The present state of the art is known to use a thermomolding material as the basis for manufacturing specific containers for food products, in a manner that the sheets in said material present, on one of their faces, a specific design consisting in a combination of colour strips; the container thus formed presents on its exposed face the same ornamental colours as those of the starting sheet in a way that, the containers being formed by portions of the starting sheet, not all of them necessarily reveal an identical design, this being a function of the selected sheet.
Said thermomolding material sheets consist mainly of extruded film of a specific thickness provided with longitudinal colour strips distributed as necessary throughout the surface of one of its sides.
The manufacture of an extruded sheet liable to be used according to the above object presents several drawbacks which heretofore have not been adequately resolved. Thus, manufacturing methods are known to provide a sheet with a surface having longitudinal colour strips, starting from a base sheet that is subsequently covered with another sheet containing the desired colours, both sheets being bonded together by thermic welding. In other cases, a plurality of parallel longitudinal strips of different colours are bonded to a base sheet in order to achieve the desired purpose. These procedures require a considerable time and labor expenditure which results in very high production costs.
French patent No. 85/15621 provided a partial solution to the problems involving the manufacture of a sheet of this kind by developing a procedure whereby a multiple sheet formed of three different layers is extruded through a single row of multiple channels, the upper layer presenting a plurality of colour strips. In this case, the widths of the colour strips are adjusted by controlling the flow of material deposited on each respective strip. The system, however, did not serve to entirely avoid the mixing of colours in adjacent strips as a result of material migrating from one strip to the other, this defect naturally being liable to affect the final product manufactured with said thermoplastic material.
Other procedures are known which allow the desired adjacent strips to be obtained without severe problems involving the migration of colours from each strip to the adjacent strip, although the snag consists in that, in the event of changes in the manufacturing process, as for example when the strips are re-distributed, the operation requires that the machine be stopped for a lengthy period of time, furthermore involving the replacement of parts of a considerable size and cost, several such parts having to be available from stock, as necessary in each case.
It would therefore be desirable to obtain a coextruded sheet which, while eliminating the problems present in the prior art, provides a perfect finish that is free of undesirable migrations, allows for proper thickness and width adjustments in each strip and also enables fast, cost-effective production. This is one of the objects of the invention.
Furthermore, a second object of the invention consists in providing elements which, once conveniently incorporated to the extrusion head, allow for changes in the distribution of the colours strips in a minimal time and at a considerably lower cost than that of prior art devices.